1. Field of the Invention
This invention relates to an image pickup apparatus. Specifically, the image pickup apparatus adds image signal output repeatedly read from an image sensor of the image pickup apparatus whenever the image signal output is read out and generates an added image. Further, when the added image is generated, the image pickup apparatus can prevent color differences in the added image due to a black level error of an optical black signal level from an image pixel in an optical black region.
An image pickup apparatus has an image sensor, such as a CCD (charged-coupled device) and a CMOS (complimentary metal oxide semiconductor device), where a photodiode is provided in each pixel. The image sensor photo-electrically converts an object image to produce an image signal. It is known that a photodiode in a pixel of an image sensor has a dark current noise signal, and consequently an output voltage offset to a photoelectric conversion circuit which converts photoelectric current of the photodiode into voltage, even in a completely light-shielded state. Therefore the image sensor is commonly provided an optical black (hereinafter referred as to “OB”) pixel region where light is physically shielded, in addition to an effective pixel region where an object image is formed. Output compensation of the image signal to remove dark current noise is performed by subtracting an average value of OB output levels calculated using OB pixel region output from photoelectric conversion output obtained from the pixels of the effective pixel region (Refer to JP2003-110942A, for example).
A destructive read-type image sensor is generally used as the image pickup apparatus in a digital camera. An accumulation level of a charge accumulating unit is reset after accumulated charge of a photodiode is read out. In this case, an image signal which has been accumulating until that moment during long-time exposure, such as bulb shooting, is displayed so that an image signal exposed at a predetermined time interval is read out first, and image signal addition is performed whenever the image signal is read out. Next, an added image obtained in this way is displayed on a display unit (Refer to JP2005-117395, for example).
With respect to the image pickup apparatus using the above-described techniques, OB compensation is performed from the image signals as described above to obtain the added image of bulb shooting, and the like. However, a slight error of the OB image signal occurs when addition is performed. Consequently, whenever addition is repeatedly performed, these slight errors are accumulated and cause color differences to occur in the added image at the end of shooting.
The occurrence of the color differences will be described using FIGS. 20A-C. Generally, the image sensor has Bayer-array color filters to obtain three color light components of R (red), G (green), and B (blue). Object light passes through the color filters of RGB and then is photo-electrically converted by the photodiode of a pixel corresponding to each color. A weighting operation of each color output is performed based on the photoelectric conversion signal to obtain a mixed luminance signal (black and white) and a color-difference signal for color reproductivity. For example, when white light is incident, only the luminance signal is a finite value and color-difference signal is zero. At that time, proportion of the signals corresponding to the R, G and G pixels is a color balance value with respect to white light. Each image sensor has a specific proportion.
When incident light is not white, the portion of the color signals of R, G, and B pixels differs from that of white light and so the image has colors. Generally, it is necessary to adjust dynamic range of R, G, and B pixel signals. Therefore, in case of a white object, a weighting operation is performed by a white balance gain in such a manner that a ratio of quantized R, G, and B pixel signals is 1:1:1.
Take as an example a situation where R, G, and B pixel signals are output as shown in FIG. 20 A (a1). Here each R, G, and B pixel signal includes OB values of the effective pixel region. If image compensation is performed by subtracting an OB pixel output value from a photoelectric conversion signal of the effective pixel region, values of the R, G, and B pixel signals are as shown in FIG. 20 A (a2). A white balance (hereinafter referred as to “WB”) gain for a B signal is g/b and that for an R signal is g/r. If the B and R signals are weighted by the white balance gain, respectively, the values of the R, G, and B pixel signals become the same as shown in FIG. 20 A (a3). The resulting image data becomes achromatic, which means that white balance processing has been properly performed.
FIG. 20 A is an example where the average value of OB output levels is equal to the OB pixel value of the effective pixel and a proper value has been used. However, as shown in FIG. 20 B, if the average value of OB output levels and the OB pixel value of the effective pixel differ each other and the average value of OB output levels includes an extra OB value “A” for the OB value of the effective pixel (Refer to FIG. 20 B (b1).), white balance processing is not properly performed. Specifically, if OB compensation is performed by a larger average value of OB output levels than the OB value of an effective pixel in the object image which is actually taken, the R and B signals from the effective pixel region become smaller than they are (Refer to FIG. 20 B (b2).). If a weighting operation is performed by a WB gain and white balance is corrected in this situation, the G signal becomes too large relative to that of the B and R signals as shown in FIG. 20B (b3), and the entire image becomes greenish.
Alternatively, as shown in FIG. 20C, if the average value of OB output levels is smaller than the OB value of the effective region by an OB value “B” (Refer to FIG. 20 C (c-1).) (that is, if OB compensation is performed by a smaller average value of OB pixel levels than the OB value of an effective pixel in the object image which is actually taken), the R and B signals from the effective pixel region become larger than they are (Refer to FIG. 20C (c2).). If a weighting operation is performed by a WB gain and white balance is corrected in this situation, the B and R signals become too large relative to that of the G signal as shown in FIG. 20C (c3) and the entire image becomes magenta.
The average value of OB output levels is an average value of output levels of a plurality of the OB pixel regions which are disposed on the image sensor and completely light-shielded, and consists of an integer part and a decimal part. Generally, OB compensation is performed by subtracting the integer part of the output level in the OB pixel region from that of the output level in the effective pixel region. A value of the decimal part corresponds to the OB value “A” or the OB value “B” in FIGS. 20 B and C, respectively. Color differences in an image are caused by the value of the decimal part because only the integer part is used for OB compensation. If OB compensation is repeatedly performed to obtain the added image, especially during bulb shooting, slight errors caused by ignoring the decimal parts during OB compensation are accumulated and then color differences become noticeable.
The invention aims at providing an image pickup apparatus which has a function of displaying an image which has been accumulating until that moment, and preventing color differences in the added image in long-time exposure such as bulb shooting.